The Baggies, a team who have used the vast advantage of parachute payments to assemble a team of long-ball, set-piece obsessives, required a 94th goal from giant centre-back to Semi Ajayi to defeat plucky Posh.
On territory and possession it was hard to deny the Baggies were worthy winners, but Posh defended with so much passion, determination and organisation they had their illustrious visitors playing frantic football throughout the second half.
Posh finally buckled in the penultimate minute after clearing yet another long throw. The ball was returned, Martin Phillips outstripped Dan Butler and sent over the perfect cross for Ajayi to convert from close range.
The visiting fans erupted and pumped-up manager Valerien Ismael was cautioned after a ridiculous run down the touchline in celebration. Ismael was seen exchanging angry words with both Posh boss Darren Ferguson and assistant boss Mark Robson after the final whistle. Presumably he it was a relief of stress after his side’s one-dimensional football looked like coming up short for the first time in four matches.
This match was very much a free hit for Posh. Expectations were low against a side playing Premier League football last season and the team news didn’t raise hopes among the home faithful.
Attacking star Siriki Dembele was left on the substitutes’ bench as Sammie Szmodics and Jack Marriott were preferred starters. Ferguson also sprang a surprise defensively as Josh Knight was dropped to the bench thus making the side smaller against a team renowned for a high intensity, set-piece loving gameplan. Butler lined up as a third centre-back.
There was a boost in the form of Jack Taylor on the substitutes’ bench for the first time this season.
And, although quality was scarce on both sides, Posh fans were encouraged by the battling qualities their side showed in the opening 45 minutes.
Frankie Kent did have to block an early Matt Phillips strike from the edge of the penalty and Christy Pym covered his angles well to beat out a Karlan Grant strike midway through the half, but Posh generally protected the penalty area well. They also gave away just one corner (it was taken short and wasted) and coped comfortably with a handful of long throws.
Posh were pushed back for a lot of the half, but the organisation was strong with wingbacks Harrison Burrows and Joe Ward containing two dangerous opponents playing in the same position.
Posh did manage a couple of promising moves. Ward had an early cross cleared after a fine run and Jonson Clarke-Harris, who played very physically, headed another Ward cross over.
An excellent sweeper-keeper in Sam Johnstone helped snuff out balls over the top, but Marriott did wriggle past Ajayi into the penalty area before placing his shot too close to Johnstone.
Another quick break saw Marriott find Burrows on the edge of the area just before the break, but his shot was deflected wide.
Taylor emerged for the second-half in place of Oliver Norburn who had been cautioned early in the game. Big centre-back Kyle Bartley also left the pitch for the visitors.
Posh survived an early scare when Pym couldn’t quite grasp a cross, but recovered to knock the ball away from Phillips.
Posh then enjoyed a four on three break after Clarke-Harris found Taylor splendidly, but the sub ignored the best option of sending Burrows clear and played a straight ball to Marriott instead who then failed to find Burrows.
The Baggies turned the screw for a 15-minute spell as throw-ins and corners rained down on Posh, but the defending was excellent from the back three and Clarke-Harris who headed one goalbound header away from danger.
Pleasingly Pym came and punched another one clear before he made a fine save to thwart debutant substitute Jordan Hugill’s close range header. Ajayi slammed the rebound against the post when he really should have scored.
But Posh weathered the storm and delivered some bright moments going forward. Ward had the beating of his new marker Adam Reach and one ferocious cross had to be well defended by Dara O’Shea.
Hugill had a second headed opportunity at the far post and then the Baggies were denied what looked like a nailed on penalty after a terrific length of the field break, but referee James Linington waved appeals away after a sandwich by Posh substitute Josh Knight and Thompson and TV replays looked to back the official up.
Posh now looked certain to claim a mightily impressive point, but for the second successive home game there was very late agony.
Posh: Christy Pym, Harrison Burrows, Dan Butler, Frankie Kent, Nathan Thompson, Joe Ward, Oliver Norburn (sub Jack Taylor, 46 mins), Jorge Grant (sub Josh Knight, 80 mins), Sammie Szmodics, Jonson Clarke-Harris, Jack Marriott (sub Siriki Dembele, 60 mins).
Unused subs: Will Blackmore, Ronnie Edwards, Joe Tomlinson, Joel Randall
West Brom: Sam Johnstone, Dara O’Shea, Kyle Bartley (sub Adam Reach, 46 mins), Semi Ajayi, Darnell Furlong, Jake Livermore, Alex Mowatt, Conor Townsend, Grady Diangana (sub Jordan Hugill, 57 mins), Kyle Phillips, Karlan Grant.
Unused subs: Alex Palmer, Caleb Taylor, Taylor Gardiner-Hickman, Robert Snodgrass, Tom Fellows.
Goals: West Brom - Ojayi (90 + 4 mins).
Cautions: Posh - Norburn (foul), Thompson (foul).
West Brom - Johnstone (delaying the restart), Mowatt (foul), Furlong (delaying the restart).
Referee: James Linington 8
Attendance: 10,163 (2,071 West Brom).